American Isner, seeded ninth in Melbourne, was beaten 7-6 (7-4) 7-6 (8-6) 6-7 (4-7) 7-6 (7-5) by compatriot Opelka in just over three hours.

The victory was Opelka's first in the main draw of a Grand Slam.

Earlier, 17-time Grand Slam champion Rafael Nadal beat James Duckworth 6-4 6-3 7-5 to reach the second round.

Opelka - who at 6ft 11ins is the joint-tallest player on the ATP Tour - is the first American to beat 6ft 10ins Isner at a Slam.

Spanish second seed Nadal, 32, saw off Australian Duckworth in two hours and 16 minutes on Rod Laver Arena, despite a stubborn finish from Duckworth.

"He was playing super aggressive, every shot," Nadal said.

South African fifth seed Kevin Anderson also progressed by beating France's Adrian Mannarino 6-3 5-7 6-2 6-1.

The 32-year-old, who has reached the fourth round at Melbourne Park on three occasions, served 14 aces but made 38 unforced errors as he won his opening match in two hours and 53 minutes.

The Wimbledon runner-up faces American Frances Tiafoe in the next round.

Nadal, who cut short his 2018 season with an abdominal muscle injury and to have ankle surgery, appeared to be back to full fitness against wildcard Duckworth.

The world number two showed no sign of being bothered by a thigh strain that had forced him to pull out of the Brisbane International earlier this month.

"I am trying to be better and better every day," he said. "I had a good week of practice before this tournament."

He will play either Australian Matthew Ebden or Germany's Jan-Lennard Struff in the second round.

Rafael Nadal won the Australian Open in 2009 but lost in the 2012, 2014 and 2017 finals